I get what you mean. When I played cyberpunk 2077 on my PC I noticed a huge performance benefit moving it from my spinning disk to my SSD. I think it's because it dynamically loads as you play. So driving a car fast down the road for example leads to a lot of distant objects loading very late.

But on my steam deck I play all kinds of other titles. Even play coop Elden ring and that runs fine but the load times definitely lack compared to my desktop 🤣.

So I think YMMV depending on the game. I've got a 512GB deck with a 1TB SD card, SD card works pretty all right.